June 18 (Reuters) - The U.S. Southwest baked under an unrelenting heatwave for a fifth day on Friday, putting power systems to the test as more records were expected to be broken before a moderating trend sets in next week.

The National Weather Service issued excessive heat warnings for five states - California, Nevada, Utah, Arizona and part of Colorado - where the heat has been extreme even for a region where temperatures often exceed 100 degrees Fahrenheit (38 degrees Celsius).

"Every year it's hot in the Southwest," said meteorologist Bob Oravec of the National Weather Service Weather Prediction Center in College Park, Maryland. "It just seems to be more newsworthy when you have temperatures of 115 or so day after day. It's pretty hot."

Among the cities where record highs were forecast on Friday was Phoenix at 117 degrees (47 degrees Celsius).

“It’s miserable, you literally just don’t leave your house unless you have to,” said Hannah Knight, 20, a server at The Coronado coffee shop in Phoenix.

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The coffee shop has an outdoor dining area but “when it reaches over 110 (degrees Fahrenheit), there’s no way to make it comfortable,” she said.

RECORD POWER DEMAND

The heatwave stressed power systems in Texas and California this week, but they withstood the record demand.

Power grid operators in both states, however, continued to urge consumer to conserve energy to reduce strain on the electric system and avoid outages as homes and businesses kept air conditioners cranked up.

In Texas, where temperatures have moderated, demand hit a record on Monday, according to the Electric Reliability Council of Texas (ERCOT), which operates the grid in most of the state.

California's power demand peaked on Thursday at 41,364 megawatts and was expected to come in close to that level on Friday, according to the California ISO, which operates the grid in most of that state. One megawatt can power about 200 homes on a hot summer day.

PEAK TEMPERATURES

State capital Sacramento was forecast to hit a new daily high 108 of degrees (42 degrees Celsius).

Many other cities were expected to come close to tying or breaking their daily records, including Las Vegas whee a high temperature of 114 degrees (46 degrees Celsius) was forecast.

The heatwave extended to the Midwest, prompting the weather services to issue advisories for Kansas, Missouri and Illinois, before a strong cold front brings relief by the end of the weekend. Temperatures in St. Louis and Kansas City, Missouri, were forecast to top 100 degrees on Friday.

Relief, relatively speaking, is also forecast to come to the Southwest at the beginning of next week, Oravec said.

"Basically, you're going to have temperatures in the low hundreds, but definitely a cooler trend, a little bit, as we go into early next week," he said.

(Reporting by Scott DiSavino and Peter Szekely in New York and Andrew Hay in Taos, New Mexico; Editing by Chizu Nomiyama and Marguerita Choy)